258 



PHYSIOGRAPHY. 



[chap. 



Fig. 73.— Diagrammatic section of an island surrounded by a fringing-reef. 



Fio. 74. — Diagrammatic section of an island surrounded by a barrier-reef, with 

 intervening lagoon. 



Fig. 75. — Diagrammatic section of a coral island, or atoll, with central lagoon. 



